The present invention is directed generally to optical transmission systems. More particularly, the invention is directed toward optical transmission systems including in situ characterized and calibrated optical amplifiers.
Digital technology has provided electronic access to vast amounts of information. The increased access has driven demand for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems to link the processing equipment.
In response to this demand, communications service providers have turned to optical communication systems, which have the capability to provide substantially larger information transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal format analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.
Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in separate waveguides, i.e. fiber optic strand. The transmission capacity of optical systems was increased by time division multiplexing (TDM) multiple low bit rate, information signals into a higher bit rate signals that can be transported on a single optical wavelength. The low bit rate information carried by the TDM optical signal can then be separated from the higher bit rate signal following transmission through the optical system.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using electromagnetic waves having different wavelengths in the optical spectrum, i.e., far-UV to far-infrared. The pluralities of information carrying wavelengths are combined into a multiple wavelength WDM optical signal that is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems were not initially deployed, in part, because of the high cost of electrical signal regeneration/amplification equipment required to compensate for signal attenuation for each optical wavelength throughout the system. The development of the erbium doped fiber optical amplifier (EDFA) provided a cost effective means to optically regenerate attenuated optical signal wavelengths in the 1550 nm range. In addition, the 1550 nm signal wavelength range coincides with a low loss transmission window in silica based optical fibers, which allowed EDFAs to be spaced further apart than conventional electrical regenerators.
The use of EDFAs essentially eliminated the need for, and the associated costs of, electrical signal regeneration/amplification equipment to compensate for signal attenuation in many systems. The dramatic reduction in the number of electrical regenerators in the systems, made the installation of WDM systems in the remaining electrical regenerators a cost effective means to increase optical network capacity.
WDM systems have quickly expanded to fill the limited amplifier bandwidth of EDFAs. New erbium-based fiber amplifiers (L-band) have been developed to expand the bandwidth of erbium-based optical amplifiers. Also, new transmission fiber designs are being developed to provide for lower loss transmission in the 1400-1500 nm and 1600-1700 nm ranges to provide additional capacity for future systems.
Raman fiber amplifiers (“RFA”) are also being investigated for use in wide bandwidth, e.g., 100 nm, optical amplifiers. RFAs are well known, but have not been deployed in commercial systems because significant pump powers on the order of hundreds of milliwatts are required to achieve relatively small levels of amplification. In addition, the RFAs that were developed did not provide a flat gain profile and thus encountered the same limitations as EDFAs. See Rottwitt et al., “A 92 nm Bandwidth Raman Amplifier”, OFC '98, p. 72/CAT-1. Despite the negatives, RFAs provide have appeal as a viable option for next generation optical amplifiers, because RFAs provide low noise, wide bandwidths, and wavelength flexible gain.
Applicants, along with co-inventors, have demonstrated that RFAs can be designed to provide controllable Raman gain profiles over arbitrary bandwidths. Raman amplifiers embodying the Applicant's invention are described commonly assigned U.S. patent application Ser. Nos. 09/119,556 and 09/253,819, which are incorporated herein by reference. The RFAs can be deployed in existing fiber optic networks having various fiber designs and compositions and over a wide range of signal wavelengths.
Recent theoretical analyses by Rottwitt et al. have confirmed Applicant's invention that multiple pump wavelengths can be used to provide a substantially flat Raman gain profile in a silica fiber over wide bandwidths. The laboratory testing and theoretical simulation results enabled a substantial decrease in the variations in the gain profile observed in their earlier studies. See Kidorf et al, “Pump Interactions in a 100-nm Bandwidth Raman Amplifier”, IEEE Photonics Technology Letters, Vol. 11, No. 5, pp. 530-2 (May 1999).
While laboratory and simulation testing is helpful, the actual performance of RFAs will generally vary depending upon the in-line, or in situ, condition of the transmission fiber, particularly for distributed and remote amplifiers. Therefore, the actual performance of the amplifiers and the transmission system can not be characterized before the deployment and operation of the system. Unfortunately, the development of optical systems having increased capacity and longer transmission distances depends on having a well characterized and controlled transmission system. It is, therefore, essential that optical systems and optical amplifiers be developed having in situ characterization and control capabilities to meet the requirements of next generation optical systems.